A control and protection device for an electric system

Abstract: A control and protection device for an electric system (S) powered from a power distribution line (L) through a switch (G) is disclosed. To avoid hazard for the system users and possible damage to the system itself, the device includes means (A,M) of inhibiting closure of the switch (G) and circuit means (E), connected to the input terminals (p and q) of the electric system (S) and effective to measure at least one electric parameter of the system in the open state of the switch and to operate the inhibit means (A,M) according to the value of the parameter measured.

Description: This invention relates to electric systems, and in particular to a control and protection device for an electric system powered from a power distribution line via a switch. Electric systems and their users need, as a matter of course, to be protected against the occurrence of various potentially dangerous situations, such as short-circuits, overloads, and insulation failures to ground or to any conductors with which a user might come in contact. To meet this requirement, protection devices have been developed which are usually incorporated to the disconnect switch through which the system is linked to a power supply line, which devices are effective to detect such danger situation and act on the switch to open its contacts. Such devices have a drawback inherent to their very principle of operation, that is they can only detect any failures likely to result in the above-mentioned danger situations while the system is in a live state. Especially on the occasion of an insulation failure or a short, this may become a serious danger for a person that, in operating the system, should touch a non-insulated appliance or close the switch in a shorted load condition. Document EP-A-0149046 discloses a control and protection device according to the preamble of claim 1, which device operates a coil driven switch connecting a network to the mains in dependence on the condition of a network. The device comprises an impedance measuring device, a processing unit and a source of a measuring voltage. The impedance measuring device has current sensors to sense currents in the phase wires of the mains and to detect any currents from the phase wires to ground. The processing unit compares the measured impedance values with reference values and provides corresponding output signals to the switch coil to open the switch if a dangerous condition is detected in the network. The object of the invention is to provide a control and protection device of this kind which is suitable for a manually operated switch. The object of the invention is achieved by a control and protection device as defined in claim 1. The invention will be more more clearly understood from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawing, the one Figure whereof shows, in substantially block diagram form, a device according to the invention as coupled to a disconnect switch. Specifically, a bipolar disconnect switch G, e.g. a differential switch protected against short circuits and overcurrents, is connected by its input terminals m and n to a line L from the AC mains, and by its output terminals p and q to an electric system S to be powered. The switch G has a pair of contacts F which can be opened or closed by means of a conventional manual operation lever, not shown. Coupled to the switch G is a control and protection device, generally denoted by the reference character D, according to the invention. This device comprises a means of inhibiting closure of the switch in the form of a low-voltage release A, e.g. that described in Italian Patent No. 1,216,585 to Bassani Ticino, and an electronic circuit E having two terminals s and v connected to the system S and a third terminal r connected to a ground clamp. The low-voltage release A is powered from the line L and consists basically of an electromagnet, a moving armature operated by the electromagnet, and a mechanical link member, designated M, between the moving armature and the switch contacts F. The circuit E is powered from the line L through a safety transformer T, or a battery B1 in the event of power outage on the line. A changeover switch S1, controlled to "make" by the line voltage, is conventionally arranged to serve this function. The electronic circuit E, shown in a greatly simplified block diagram form in the drawing, basically comprises a set EI of three current meters A1, A2, A3, a changeover device SW represented by three switches SW1, SW2, SW3 and a changeover switch SW4, and a timer CLK. The timer controls the changeover device such that the three switches SW1, SW2 and SW3 will be closed alternately and consecutively and that the changeover switch SW4 set to the closed contact position to the terminal connected to the ground clamp when the switches SW1 and SW2 are closed, and to the other position of closed contact to the terminal S when the contact SW3 is closed. The output terminal z of the circuit E, being the output of the set EI, is connected to the control terminal of a switching device P which has a normally closed contact K connected in one of the power supply leads to the release A. The device P may be a solid-state static switch or an optoelectronic coupler providing galvanic isolation of the system S from the line L upon the switch G "breaking". In this embodiment, the device D includes a module UC connected to the circuit E for remote controlling and/or signalling, as well as optical and sound indicator members H, I to display certain operating conditions of the device. The device operation will now be considered in the condition where the switch contacts F are open. The circuit E performs some measurements of certain electric parameters of the system. In particular, it applies, to the input terminals of the system S, a safe low-value test voltage, conformant with the safety regulations for electric systems, which voltage is supplied from the secondary winding of the transformer T or the battery B1, and measures the current being caused to flow through the system by this voltage. In the closed state of the switch SW3, if the current measured does not exceed a predetermined safe value represented by the current Ir1 in the drawing, no signal is issued on the output z of the circuit E, and the switch K stays closed. Under this condition, the low-voltage release A is powered as normal and will not interfere with the switch G which can, therefore, be closed by operation of its manual control lever. Conversely, where the current measured exceeds the predetermined value, this is understood to mean that the system impedance has dropped below an acceptable safe limit, e.g. due to a failure in a load connected to the system, and the circuit E will act on the device P to open the contact K. The low-voltage release A is no longer under power, and accordingly, will act on the switch G through the mechanical link member M to lock its contacts F to the open position. Another measurement performed by the circuit E consists of finding out any ground failures. This is carried out with the switch SW1 or SW2 in the closed state. Here the test voltage is applied between either terminal s or v and the ground clamp. If the current measured by either of the current meters A1 and A2 is below a second reference current Ir2, the circuit E issues a signal on its ouput z to so control the device P as to open the contact K, and accordingly, inhibit closure of the contacts F of switch G. Where the contacts F of switch G are closed, the existence of a malfunction in the system would cause the automatic release devices of the switch to be operated, the switch being responsive, as previously mentioned, to short circuits, overloads, and current leaks to ground. Under this condition, the electronic circuit E can be deactuated to only retain a signalling function through the indicators H and I and/or the module UC. As can be seen, having the circuit E powered through a safety transformer T and the circuit E coupled to the low-voltage release A through an optoelectronic device ensures full galvanic isolation between the line L and the system S in the open state of the switch. With the line voltage out, the release coil A would be powered from the battery B2 and left under power for a predetermined time period which may be the battery discharge time. In view of the inventive device allowing a system to be tested even with no voltage from the power distribution line, it can advantageously be used to ascertain the operability of a newly installed system, prior to applying a normal operating voltage thereto. The remote control and signalling module UC allows the circuit operation to be controlled from a remote station, e.g. to receive actuation commands for the circuit E, commands to change the test values, etc., or to transmit information about measurements, any alarm state, etc. to the remote station. As can be appreciated from the foregoing, the device of this invention does overcome the drawbacks of the prior art by enabling diagnostics of the system in the open switch state, that is in a fully safe manner for both the user and the system itself, using a safety source of a low voltage. Full safety is also ensured by that the test circuit is powered from the line through a safety transformer or a battery. While one embodiment only of the invention has been described and illustrated, many changes and modifications may be made thereunto within the scope of the inventive concept defined by the appended claims. For example, instead of using a low-voltage release as described, any other electromagnet device could be used; the electronic circuit E could be arranged to detect any ground failures without material connection to a ground clamp, or in such a manner that the various system tests can be carried out continuously and at one time, rather than consecutively; the device P could be a magnetic, capacitive or another type of coupling device, instead of an optolelectronic device. Finally, the device of this invention may be fabricated and enclosed within a separate case for attachment to the disconnect switch, or may be incorporated to said switch.

Claims: A control and protection device for an electric system (S) powered from a power distribution line (L) via a switch (G), said control and protection device comprising means (A,M) for inhibiting closure of the switch, and circuit means (E), connected to the input terminals (p,q) of the electric system (S) and to a power source (L,B1), effective to measure at least one electric parameter of the system in the open state of the switch and to operate the inhibit means (A,M) according to the parameter measured, characterized in that the inhibit means (A,M) comprises an electromagnet (A), a moving armature actuated by the electromagnet, and a mechanical link member (M) between the moving armature and the switch (G) which prevents the switch from being closed upon operation of the inhibit means. A device according to Claim 1, characterized in that the power source (L,B1) comprises a safety voltage source (T,B1) being connected, for the measurement, to the electric system (S) through the circuit means (E), and that the system (S) own impedance is an electric parameter for measurement. A device according to Claim 1, characterized in that the power source (L,B1) comprises a safety voltage source (T,B1) being connected, for the measurement, to the electric system (S) through the circuit means (E), and that the system insulation from ground is an electric parameter for measurement. A device according to Claim 1, characterized in that the power source (L,B1) comprises a safety power source (T,B1) being connected, for the measurement, to the electric system (S) through the circuit means (E), and that the parameters for measurement are the system own impedance and insulation from ground. A device according to Claim 4, characterized in that the two electric parameters are measured consecutively. A device according to Claim 4, characterized in that the two electric parameters are measured continuously and simultaneously. A device according to either Claim 2, 4, 5 or 6, characterized in that the circuit means (E) comprises means (ET,SW) for measuring the current flowing through the system when the safety voltage source (T,B1) is connected thereto, to compare that measured current with a first predetermined reference current (Ir1) and operate said inhibit means (A,M) if the measured current exceeds said first reference current (Ir1). A device according to any of Claims 3 to 7, characterized in that the circuit means (E) comprises means (EI,SW) for measuring the current flowing between a ground terminal and each of the system input terminals (p,q) when the safety voltage source (T,B1) is connected thereto, to compare that measured current with a second predetermined reference current (Ir2) and operate said inhibit means (A,M) if the measured current exceeds said second reference current (Ir2). A device according to any of the preceding Claims, characterized in that the electromagnet (A) is connected to a power source (L,B2), and that the inhibit means (A,M) is operated with the electromagnet (A) in the off state. A device according to Claim 9, characterized in that the power source is the power distribution line (L) ahead of the switch (G), and that it comprises a switching device (P) adapted to be operated by the circuit means (E) to break the connection between the electromagnet (A) and its power source, thereby operating the inhibit means (A,M). A device according to Claim 10, characterized in that the switching device (P)is a solid-state static switch. A device according to Claim 10, characterized in that the switching device (P) is an optoelectronic coupler. A device according to Claim 9, characterized in that the power source for the electromagnet (A) is a battery (B2). A device according to any of the preceding claims, characterized in that the safety voltage source comprises a transformer (T) connected to the power distribution line (L) ahead of the switch (G). A device according to any of Claims 1 to 13 above, characterized in that the safety voltage source comprises a battery (B1). A device according to any of the preceding claims, characterized in that it includes a remote control and signalling means (UC) connected to the circuit means (E) and effective to link said circuit means with a remote station for receiving and/or transmitting data.

Inventors: FABRIZI FABRIZIO, FABRIZI, FABRIZIO
Applicants: BTICINO SPA, BTICINO S.P.A.
Application year: 1994
Main classification: H02H11
IPC codes: H02H11